As a serious answer:
GT5 was not a complete game, and didn't ever become one, even after it's updates, in my opinion. There were 800+ cars and several tracks that looked like they were directly ported from a PS2 game (partially because they sorta were...), A-Spec was disorganized and short, the tack "creator" was really just a "generator" over "random" terrain maps, online was incredibly buggy, and many systems in the game (paint, upgrades, car maintenance, etc.) were flawed. These are the things I can list just off my head.
All of this considered, I don't believe that would mean that GT5 should be really called "GT4.5" or something along that line. It was massively different from GT4 (you could now roll your car over, damage your car, "build" tracks, race online, and drive in varying time and weather conditions, for an example).
GT6 will more than likely not be a complete game at launch, but that doesn't mean it will be a "GT5.5". We've already seen that the physics are having a MAJOR upgrade in several ways, which already begins to set it's self as a new game. It appears that the career/A-Spec is being restructured, along with car purchasing and customization. New modes are being introduced, and online is supposed to be revamped. Night and weather racing is also improved. I don't think GT6 will TRULY be "GT6" at launch, as B-Spec and the track creator won't be there on day one. But that doesn't make it an evolved GT5, as I just explained. GT5 was already meeting the limits graphically, so no real breakthroughs should've come there - yet they sort of have now, with the new features in night racing and the lighting engine.
I really can't call it GT5.(x) - because it really IS a new game. I think at launch the game will not quite live up to it's title, but once the track creator and other anticipated features (as in features we've heard about but have been delayed for a future update) are put in, I think the GT6 title will be lived up to.
So, GT6, in my opinion, really IS GT6.